Synthesis, magnetic properties, and STM spectroscopy of an unprecedented octanuclear chloro-bridged nickel(II) double cubane

Reaction of nickel(II) chloride hexahydrate with N-n-butyldiethanolamine H2L (3) in the presence of LiH in anhydrous THF leads to the formation of the unique octanuclear chloro-bridged nickel(II) double cubane [({NiII4(μ3-OH)Cl3(HL)3}μ2-Cl)2] (4) in 57% yield. According to single crystal X-ray structure analysis, complex 4·4CH2Cl2 possesses a [({Ni4(μ3-OH)(μ3-O)3(OH)3(N)3(Cl)3}μ2-Cl)2] core and crystallizes in the monoclinic space group P21/c with a = 18.292(2), b = 19.8972(5), c = 23.295(2) A, β = 98.408(6)°, V = 8387.3(8) A3, and four molecules in the unit cell. The analysis of the SQUID magnetic susceptibility data identified 4 as a weakly coupled dimer (J1 = 14.5 K, J2 = −0.6 K) with a ground state of S = 0, resulting from two S = 4 states of each {Ni4} subunits. Although complex 4 does not show an ac out-of-phase signal in a zero dc field at temperatures of 1.8 K and higher, low-temperature magnetization measurements revealed that complex 4 is a single-molecule magnet and shows hysteretic magnetization characteristics with typical temperature and sweep-rate dependencies. The eye-catching feature of complex 4 is the presence of two different blocking temperatures (0.9 K around zero field and 1.3 K at higher fields). The origin of this highly unusual behavior can be assigned to the dimer-nature of the interaction between the two S = 4 units. Furthermore STM and current imaging tunnelling spectroscopy (CITS) were performed on aggregates of 4 drop-coated on highly oriented pyrolytic graphite (HOPG) surfaces. CITS measurements show a strong contrast in the area of the nickel centers and a HOMO–LUMO gap of approximately 0.8 V.